This invention relates to the recovery and purification of iridium with reference to its removal from solutions containing rhodium and impurities such as base metals.
In this specification the term "base metal" refers to any metal impurity other than a member of the platinum group of metals.
Iridium is normally recovered together with other platinum group metals in the form of a platinum metal concentrate but generally forms a very minor constituent thereof. A typical ratio of platinum group metals to one another in such a concentrate would be:
Pt: 1 PA1 Pd: 0.5 PA1 Ru: 0.2 PA1 Rh: 0.1 PA1 Os, Ir: 0.02 PA1 1. Commercial solvents almost always contain impurities or, are themselves, capable of reducing Ir(IV) to Ir(111) which may then be back-extracted albeit to a minor extent. Attempts have been made to counter the back extraction by maintaining highly oxidising conditions in the aqueous phase during extraction for example, by saturating the solution with chlorine. Such techniques tend to cause solvent degradation and are not easy to implement in large scale solvent extraction equipment. PA1 2. Oxidation of Ir.sup.(111) to Ir.sup.(IV) in hydrochloric acid medium does not imply complete conversion to the extractable complex IrCl.sub.6.sup.2-. Ir.sup.(111) is usually found as a mixed, aquo-chloro complex of the form Ir(111)Cl.sub.x (H.sub.2 O).sub.y - (x-3), where x + y = 6. Oxidation of Ir(111) in HCl solution thus usually produces a mixture of Ir(IV) chloro-aqua complexes of similar form and having a charge of (x - 2). Because of the kinetic inertness of Ir(IV) interconversion of such complexes does not readily occur and as the full chloro-complex is the only one completely extracted by most organic solvents, complete extraction is thus not achieved. Where virtually complete extraction of iridium has been claimed in the literature, it is usually found that the starting solution has been prepared from an Ir(IV) salt which does not at all resemble a solution obtained by oxidising Ir(111) in solution. PA1 1. The iridium containing solution is made acidic with HCl, the preferred HCl concentration being about 6M. The iridium is then oxidised to the +IV oxidation state at a temperature of about 40.degree. C. Chlorine is the preferred oxidant. PA1 2. The oxidised solution is passed through a bed of ion exchange resin at a flow rate of not more than 1 ml/cm.sup.2 /min. A strong base resin is preferred for this operation because of its resistance to oxidation. Amberlite IRA-400 (a Rohm & Haas product, sold under the Trade Mark Amberlite), is an example of a suitable commercially available resin. PA1 3. After loading, the resin is washed with weak (0,1M) HCl to remove entrained feed solution, excess acid and weakly absorbed base metal impurities. PA1 4. A saturated solution of SO.sub.2 in water is passed through the washed resin bed. Simultaneous reduction and complexing of the iridium are believed to take place, with the formation of mixed sulphito-chloro complexes of the form Ir(111) Cl.sub.x (SO.sub.3).sub.y where x is probably 4 and y is probably 2. Such complexation only takes place in the virtual absence of free hydrochloric acid. Other methods of eluting have used saturated SO.sub.2 solutions of a dilute acid, usually hydrochloric acid. In these cases the sulphite ion acts as a reductant only and elution is inefficient as the iridium species formed is IrCl.sub.6.sup.-3 which is only slowly eluted from anion exchange resin. PA1 5. The mixed sulphito-chloro complex can now be effectively and completely eluted from the resin using hydrochloric acid as the eluant. 6M HCl is the preferred eluant. The efficiency of the elution step is far superior to that obtained when simple reduction is used, and this is believed to be because the sulphito chloro complex formed in this method has a large negative charge (-5) and, as is well known, the distribution coefficient for retention of anions on anion-exchange resins is highly dependent of the charge on the anion; anions with low charge being absorbed much more readily than those with a high charge; thus IrCl.sub.6.sup.3- would tend to be much more difficult to elute than Ir Cl.sub.4 (SO.sub.3).sub.2 .sup.-5. PA1 1. Removal of the SO.sub.2 and conversion of the chloro-sulphito complex into the Ir(111) chloro-complex by boiling, and, PA1 2. Oxidation of the Ir(111) to Ir(IV) in similar manner to that described above. As no extra cations e.g. Na.sup.+ are introduced into the eluate, highly concentrated solutions of Ir suitable for solvent extraction or direct salt precipitation can be obtained.
Because of its low concentration the recovery and purification of iridium by known methods is difficult and often incomplete. These methods rely on the relative inertness of iridium to leaching in order to concentrate it into a smaller bulk with the other minor platinum group metals. It is then recovered by a series of dissolution/reprecipitation steps in which impurities are removed at each stage. The disadvantages of this process are mainly the resultant incomplete recovery of iridium at each stage which in turn necessitates recycling material to recover the iridium. Another problem with the presently used process evidences itself in the purification of the other platinum group metals, especially in the case of rhodium where iridium is often the most persistent impurity.
Many attempts to improve the recovery of iridium have been made using ion-exchange resins or solvents. All known techniques of which Applicants are aware, are based on the oxidation of Ir to the Ir(IV) oxidation state and the formation of the extractable complex Ir Cl.sub.6.sup.2-. This species is then extracted from the solution using an anion exchange resin or solvent, thus effecting a separation from metals, such as Rh which form less extractable complexes such as RhCl.sub.6.sup.3-, which do not form extractable complexes of this type. Resin ion-exchange is generally acknowledged as giving more complete extraction, while solvent extraction techniques are recognised as being more selective. Solvent extraction methods have been favoured up to now because resins have proved difficult to elute effectively, whereas stripping of solvents can easily be achieved.
Solvent extraction methods do however suffer generally from the abovementioned disadvantage that complete extraction is difficult to achieve without special techniques being involved. The reasons for the incomplete extraction associated with solvent extraction are not completely understood but the following two effects are considered by applicants to be of importance:
This difficulty may be overcome by inserting, between each extraction stage, a "conditioning" step, in which the iridium remaining in the aqueous phase is heated and oxidised so that more of the full chlorocomplex is formed and can be extracted. In this way virtually complete extraction can be achieved, but such a process is cumbersome to implement in practice.
The more complete but less selective iridium extraction by solid ion-exchange resins as compared with solvent extraction may be explained as follows:
Typical ion-exchange resins will extract not only the full chloro-complex, but also mixed chloro-aqua complexes which are anionic in character. Thus, if an Ir(111) solution containing 85% of the Ir as IrCl.sub.6.sup.3-, 10% of the Ir as IrCl.sub.5 (H.sub.2 O).sup.2- and 5% as IrCl.sub.4 (H.sub.2 O).sub.2.sup.-, is oxidised to give the corresponding proportions of the Ir(IV) complexes, a resin will extract 95% of the Ir whereas a solvent will extract only 85%. Moreover, increasing substitution with aqua groups leads to increasing lability of these groups so that the equilibrium between the mono and di-aqua complexes can be established in a reasonable time whereas no equilibrium condition is obtained between the full chloro and mono aqua substituted complexes. Therefore, in resin ion-exchange, where the mono-substituted complex is effectively removed from solution, re-establishment of the equilibrium is possible and will lead to conversion of di- to mono-substituted complexes and to further extraction. Furthermore, it is easier to maintain oxidising conditions within a resin bed in practice than is the case with solvent extraction.
Because of the usually low concentrations of iridium present in feed solutions obtained in practice, ion-exchange is a more attractive process than solvent extraction in terms of the size and ease of operation of the equipment involved.
Thus, because of better extraction, ease of maintaining oxidising conditions and ease of operation, ion-exchange is preferable to solvent extraction for iridium recovery. However, solvent extraction is definitely to be preferred in terms of selectivity with respect to, and purification of, iridium.
Furthermore as mentioned previously, stripping of the iridium from solvents can easily be accomplished by contacting loaded solvents with a reducing solution. However, such techniques are not efficient when applied to anion exchange resins.
An object of the invention is therefore to provide a process for the recovery of iridium, the process providing good selectivity and high efficiency.